Recently, a technique of monitoring the vicinity of a vehicle for the purpose of driving safety has been developed. According to this technique, an ultrasonic sensor is mounted in the vehicle, which may include an automobile. The ultrasonic sensor receives a reflected wave of an ultrasonic wave harmless to a human body, which is transmitted from the ultrasonic sensor, so as to measure the position of or a distance from an object present in the vicinity of the automobile, a two-dimensional shape, or a three-dimensional shape of the object and the like.
For example, the following automatic parking system has been put into practical use. An ultrasonic sensor is mounted in a rear part of an automobile. A device, generally called “a back sonar,” is used while reversing the automobile into a parking space to avoid the collision with an object. The “back sonar” is for detecting the object, which may include a human or another obstacle, present behind the automobile.
As an ultrasonic sensor used for the above-described usage, a piezoelectric or a capacitive (condenser) ultrasonic sensor fabricated by employing a Micro Electro Mechanical Systems (MEMS) technique is known.
For example, a technique of juxtaposing a plurality of ultrasonic sensor elements has been disclosed as a piezoelectric ultrasonic sensor employing the MEMS technique. Each of the ultrasonic sensor elements is composed of a piezoelectric sensor, which includes a ferroelectric member interposed between a pair of electrodes. The piezoelectric sensor has a predetermined resonance frequency to detect an ultrasonic wave. Such a device is disclosed in Japanese Patent Laid-Open Publication No. 2003-284182.
The ultrasonic sensor disclosed in the above publication includes a piezoelectric element, which serves as a piezoelectric sensor, formed on a semiconductor chip having a “Silicon On Insulator” (SOI) structure. The piezoelectric element includes a thin film made of a PZT (lead zirconate titanate) ceramic corresponding to a ferroelectric material interposed between two thin electrode layers including an upper electrode layer and a lower electrode layer.
Therefore, each of the electrode layers and the PZT ceramic thin film have a low mechanical strength. As a result, there arises a problem that each of the electrode layers or the PZT ceramic thin film is vulnerable to damage upon application of an external force to the upper electrode layer so that the piezoelectric element is likely to be broken.
On the other hand, the capacitive ultrasonic sensor using the MEMS technique includes: a fixed electrode layer formed on a semiconductor chip; and a thin movable electrode layer provided on the fixed electrode layer through a gap. The fixed electrode layer and the movable electrode layer form a capacitive element.
With the above structure, the movable electrode layer has a low mechanical strength. Therefore, there arises a problem that the movable electrode layer is vulnerable to damage upon application of an external force to the movable electrode layer so that the capacitive electrode is likely to be broken.
As described above, the conventional piezoelectric or capacitive ultrasonic sensors fabricated by employing the MEMS technique are likely to be damaged under the application of an external force. Therefore, it is difficult to mount the conventional ultrasonic sensor in an automobile as external equipment.